When marking with laser, a galvanometer may cause a distortion of marked pattern. Marking distortion of the galvanometer may be caused by following two reasons. On one hand, since a deflection lens inside the galvanometer is rotated to realize a linear movement within a plane and an arc is formed by the deflection on the plane, an arc pincushion distortion may be formed on the plane, as shown in FIG. 1; and on the other hand, due to an optical aberration of a field lens, a barrel distortion may be caused, as shown in FIG. 2. In fact, aberration is a superposed distortion in X and directions caused by a superposition of the two distortions. The two distortions may directly impact on the marking effect, such that the marked pattern is distorted differently according to different coordinate of each point. Therefore, in the marking industry, solution of the two distortion is regarded as an important basis to judge the marking effect.
At present, there are two distortion correction methods, which are a formula method and a look-up table method. In the formula method, first, actual coordinates of mark points obtained by marking N (N may be selected according to required precision) uncorrected images to be marked is necessarily collected, and then a correcting formula is created by fitting differential values between the actual coordinates of mark points and preset desirable coordinates of mark points. With the correcting formula, a deviation value of the coordinate of each mark point on the marking plane can be obtained. Before marking, the deviation value of the coordinate of each mark point is first calculated, and then the deviation value of the coordinate is added to the actual coordinate of mark point to obtain a corrected coordinate of mark point. Marking with the corrected coordinate may correct the distortion. However, this correction method has a lower precision and may result in a fracture of image.
In the look-up table method, the image to be marked is divided into different regions. The deviation values of mark points in each region can be obtained by correction experiment. The deviation values of mark points are saved as a coordinate deviation table. When there is a need to mark, the deviation value of each mark point can be obtained by looking up the table. The smaller region it is divided into, the higher the correction precision would be. A disadvantage of this method is in that, when there is a need to measure a deviation table with high density and high precision, operations would be complicated, time consumption would be significant and it would be difficult to measure a great number of points.